Conventional processes for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feedstocks including catalytic reformate, hydrogenated pyrolysis gasoline, etc., utilize an aromatic selective solvent. Typically, in the practice of such processes, a hydrocarbon feed mixture is contacted in an extraction zone with an aromatic extraction solvent which selectively extracts the aromatic components from the hydrocarbon feedstock, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons, and an extract phase comprising solvent having aromatic components dissolved therein.
The aromatic hydrocarbons are typically recovered from the extract phase, i.e., separated from the aromatic extraction solvent and water, by one or more distillation steps. Often, steam distillation is employed to assist in recovering the aromatic hydrocarbons from the solvent.
In contrast, the raffinate phase from the extraction zone is typically purified by water-washing. Often, the water used for washing the raffinate phase is obtained from the aqueous phase of an overhead, or side-draw, distillate from the extract phase distillation columns, i.e., condensed steam. The aqueous phase, which has low levels of solvent, is passed to one or more raffinate wash columns where residual aromatic extraction solvent is recovered from the raffinate phase. Spent raffinate wash water is passed to a steam generator along with any other solvent-containing water streams that may be present in the process to provide relatively pure steam which is recirculated to the extract phase distillation columns.
One process exemplary of the above-described water washing technique is described in U.S. Pat. No. 4,058,454. The patent discloses a solvent extraction process for separating polar hydrocarbons from non-polar hydrocarbons. The solvent-rich extract phase from the extraction zone is introduced into a stripping column. The stripper bottoms, being a polar hydrocarbon-containing, first solvent-rich stream, is introduced into an upper portion of a solvent recovery column. A vaporous stripping medium is introduced into a lower portion of the solvent recovery column. The stripping medium, i.e., water, is condensed in an overhead condensor and used as raffinate wash water.
Another process disclosed in U.S. Pat. No. 3,864,244 employs a steam generator to supply stripping steam but uses only one distillation column to separate non-aromatics from aromatics and from extraction solvent. The stripping steam is condensed with the aromatics product (drawn as a sidestream) and after phase separation is sent to the wash column. A portion of the steam passes overhead in the distillation column and is condensed with the non-aromatics. The aqueous phase of the overhead condensate is combined with the spent raffinate wash water and vaporized to generate stripping steam for the aromatics distillation. The process relates to the contacting of the spent raffinate wash water with a portion of the aromatics product to remove the non-aromatic contaminants before steam is generated.
U.S. Pat. No. 4,693,810 discloses a process wherein stripping water obtained from the steam distillation column is divided into two portions. One portion is passed to a steam generator. Another portion is vaporized by heat exchange with the lean solvent bottoms from the steam distillation column to vaporize the stripping water. The two portions are then combined and introduced as stripping steam into the steam distillation column.
It has been found that when a relatively lean feed is extracted, i.e., contains a large amount of non-aromatics, that water flowschemes such as described above can be inadequate. A large non-aromatic raffinate phase requires an unusually large amount of wash water. In the conventional process, the wash water is provided from the stripping steam, but when more water is needed than steam is required for stripping, operation of the column in this manner can be inadequate.
Accordingly, processes are sought which can provide for the recovery of extraction solvent from the raffinate phase from aromatic extraction processes when the raffinate phase is large in comparison to the extract phase, i.e., when the feed is lean in aromatic hydrocarbons.